1. Field of the Invention
The present invention relates to compositions and methods for inhibiting the formation, deposition and adherence of alkaline earth metal scale deposits, especially calcium carbonate (CaCO.sub.3) scale deposits, on metallic surfaces of aqueous systems, especially under conditions of high pH and high calcite concentration, e.g., those found in cycled up cooling tower systems, where those compositions are N,N-bis(phosphonomethyl)-2-amino-1-propanol, derivatives and corresponding lower alkyl ethers and N-oxides thereof.
Generally, calcium carbonate scale deposits are incrustation coatings which accumulate on the metallic surfaces of a water-carrying system through a number of different causes.
Various industrial and commercial water-carrying systems are subject to calcium carbonate scale formation problems. Calcium carbonate scale is of particular concern in heat exchange systems employing water, such as, for example, boiler systems, and once-through and open recirculating water cooling systems. Cooling towers are especially significant, particularly where severe conditions, including high pH and high calcite concentrations are encountered.
The water employed in these systems ordinarily will contain a number of dissolved salts, and the alkaline earth metal cation calcium is usually prevalent, as is the anion carbonate. The combination product of calcium cation and carbonate anion will precipitate from the water in which they are carried to form scale deposits when the concentration of the anion and cation comprising the reaction product, i.e., calcium carbonate, exceeds the solubility of the reaction product itself. Thus, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction product, a solid phase of calcium carbonate will form as a precipitate. Precipitation of the reaction product will continue until the solubility product concentrations of the constituent ions are no longer exceeded.
Numerous factors may be responsible for producing a condition of supersaturation for the reaction product calcium carbonate. Among such factors are changes in the pH of the water system, evaporation of the water phase, rate of heat transfer, amount of dissolved solids, and changes in the temperature or pressure of the system.
For cooling systems and similar heat exchange systems including cooling towers, the mechanism of scale formation is apparently one of crystallization of scale-forming salts from a solution which is locally supersaturated in the region adjacent the heating surface of the system. The thin viscous film of water in this region tends to become more concentrated than the remainder of the solution outside this region. Precipitation is also favored on the heat transfer surface because of the inverse solubility relationship of calcium carbonate. As a result, the solubility of the scale-forming calcium carbonate salt reaction product is first exceeded in this thin film, and crystallization of calcium carbonate scale results directly on the heating or heat exchange surface.
In addition to this, a common source of scale in boiler systems is the breakdown of calcium bicarbonate to form calcium carbonate, water and carbon dioxide under the influence of heat. For open recirculating cooling water systems, in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat by evaporation of water, the chief factor which promotes calcium carbonate scale formation is concentration of solids dissolved in the water by repeated evaporation of portions of the water phase. Thus, even a water which is not scale forming on a once-through basis usually will become scale forming when concentrated two, four, or six times. Moreover, alkalinity of the makeup water, with evaporative cycles over time results in an increasing alkalinity of the water in the overall system, often reaching pH's of 8.5-9.5 and even higher. Conventional scale inhibiting compositions typically fail in systems having such severe conditions.
The formation of calcium carbonate scale deposits poses a serious problem in a number of regards. The calcium carbonate scale which is formed possesses a low degree of heat conductivity. Thus, a calcium carbonate scale deposit is essentially an insulating layer imposed across the path of heat travel from whatever source to the water of the system. In the case of a cooling system, the retarded heat transfer causes a loss in cooling efficiency. In addition to this problem, calcium carbonate scale formation facilitates underdeposit corrosive processes, and a substantial calcium carbonate scale deposit will interfere materially with fluid flow. Consequently, calcium carbonate scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Although the present invention is directed primarily to preventing or inhibiting the deposition of calcium carbonate scale, the most prevalent type of scale deposit, it is also applicable to inhibiting the deposition of other types of alkaline earth metal scales, especially where those are associated with calcium carbonate scale under the severe conditions described herein. For example, most industrial and commercial water contains alkaline earth metal cations, such as calcium and magnesium, and several anions such as bicarbonate, carbonate, and phosphate. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. These precipitates are alkaline earth metal scales. Thus, by alkaline earth metal scales is meant scales including but not limited to calcium carbonate, magnesium carbonate, and calcium phosphate. These scales form frequently in the tubes of heat exchangers and on other heat exchange surfaces, such as those in cooling towers. Particular systems or applications areas where severe conditions lead to exceptional buildup of calcium carbonate and related scales, in addition to cycled up cooling towers, include reverse osmosis systems, sugar refining evaporators, and certain types of gas scrubbers.
The N,N-bis(phosphonomethyl)-2-amino-1-propanol, derivatives and corresponding lower alkyl ethers and N-oxides of the present invention are used in the same range of amounts as threshold inhibitors in the scale inhibition method of the present invention, rather than as sequestering or chelating agents, although the compositions of the present invention have dispersant properties as well and significantly reduce the adherency of any scale deposit which is formed, facilitating its easy removal.
Scale-forming compounds can be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Generally, this requires many times as much chelating or sequestering agent as cation, since chelation is a stoichiometric reaction, and these amounts are not always desirable or economical. However, several decades ago, it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating.
When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale-forming cation (stoichiometric), it is said to be present in "threshold" amounts. See, for example, Hatch and Rice, Indust. Eng. Chem., 31, 51-53 (1939); Reitemeier and Buehrer, J. Phys. Chem., 44 (5), 535-536 (1940); Fink and Richardson U.S. Pat. No. 2,358,222; and Hatch, U.S. Pat. No. 2,539,305.
Similarly, anionic and cationic polymers can be used as dispersants in accordance with methods known in the art, but the dosage levels necessary to achieve dispersion are in the range of 0.5-1.0% by weight of the system being treated, which is many orders of magnitude higher that the dosage levels used for the compositions of the present invention. Thus, it is a unique aspect of the present invention that it is possible to achieve essentially non-adherent scale using only threshold inhibitor dosage levels of the compositions of the present invention.
Recently, attention has been focused on controlling scaling under severe conditions, where conventional treatments such as those described above do not provide complete scale control. Current technology in scale control can be used to inhibit CaCO.sub.3 scale up to 100 to 120 times calcite saturation, i.e., a water containing Ca.sup.2+ and CO.sub.3.sup.2- present at 100 times (100.times.) their solubility limit. However, what is desired are inhibitors effective in greater than 150.times. water, especially in greater than 250.times. water, i.e., where especially in greater than 300.times. water, i.e., where the calcite ions can be prevented from precipitating as calcium carbonate scale using substoichiometric amounts of an inhibitor. The compositions of the present invention are especially useful under severe conditions characterized by a calcite saturation level of 150.times. and above, especially 250.times. and above, and more especially 300.times. and above, as defined in the paragraph immediately below.
Severity of the scaling tendency of a water sample is measured using the saturation index, which may be derived in accordance with the following equation: ##EQU1## where SI is the saturation index for calcium carbonate, [Ca.sup.2+ ] is the concentration of free calcium ions, [CO.sub.3.sup.2- ] is the concentration of free carbonate ions, and .sup.K spCaCO.sub.3 is the conditional solubility product constant for CaCO.sub.3. All of the quantities on the right side of the above equation are adjusted for pH, temperature and ionic strength.
Calculation and use of the saturation index, and generation of the data from which it is derived, are matters within the skill of the art. See, for example, Critical Stability Constants, Vol. 4: "Inorganic Complexes", Smith & Mantell (1976), Plenum Press; and Aquatic Chemistry, Chap. 5, 2nd ed., Stumm & Morgan (1981), Wiley & Sons.
Another characteristic feature of the severe conditions under which the scale controlling compositions of the present invention are especially useful is high pH, i.e. a pH of 8.5 and higher, particularly a pH of 9 or 10 or even higher. A related feature of such severe conditions is high alkalinity.
One of the particular advantages of the scale inhibiting compositions of the present invention is the exceptional calcium tolerances which they exhibit. Calcium tolerance is a measure of a chemical compound's ability to remain soluble in the presence of calcium ions (Ca.sup.2+). One of the parameters of scale control under severe conditions is pH. As pH increases, calcium tolerance decreases rapidly for traditional CaCO.sub.3 threshold inhibitors, e.g., 1-hydroxy ethylidene 1,1-diphosphonic acid (HEDP) and amino tri(methylene phosphonic acid) (AMP). These inhibitors precipitate with calcium at alkaline pH's, rendering them useless as threshold scale inhibitors. While it is common practice to use an acid feed to the water of, e.g., a cooling tower system in order to lower pH and thus avoid the calcium tolerance problem for conventional inhibitors, the danger to handlers which such acid feeding poses makes it all the more important to find scale inhibitors which operate at high pH's.
An advantage of the scale inhibiting compositions of the present invention which are N-oxides is their ability to maintain a level of resistance to degradation by oxidizing biocides which is sufficient to ensure adequate scale inhibition at dosing levels within the ranges herein described. This is of particular importance in cooling systems such as those using cycled up cooling towers. Such systems maintain a large body of water for a considerable length of time exposed to the atmosphere under conditions which do not include sufficient aeration and exposure to sunlight to provide control of microbial, especially bacterial and fungal, growth. Unchecked, such microorganisms flourish and produce colonies extensive enough to give rise to problems of biofilm blockage of heat exchange surfaces, and clogging of the components of the water transporting apparatus used in operating the cooling system.
Such problems of unwanted microbial growth in a cooling system are usually solved by use of an oxidizing biocide, especially chlorine or bromine, since these are inexpensive, effective, and produce minimal environmental impact. However, as is well known, such oxidizing biocides also tend to degrade scale inhibitors containing a N,N-bis(phosphonomethylene) group, presumably by oxidative attack on the nitrogen atom of the group. It has been found that the N-oxides of the present invention offer significant resistance to such degradation; and they will continue to provide scale inhibition when dosed in accordance with the ranges set out herein.
It is also a surprising attribute of the compounds, including N-oxides of the present invention that, even though they provide unacceptably low scale inhibition with aqueous systems having normal conditions and scaling tendencies, they provide an unexpectedly high level of scale inhibition protection in aqueous systems characterized by the severe conditions of high pH, high calcite concentration, etc., and having severe scaling tendencies, as described in detail further herein. It was wholly unexpected that compounds having that attribute, would also provide resistance to degradation by oxidizing biocides as well, under the severe conditions and scaling tendencies just described.
2. Brief Description of the Prior Art
Early efforts to reduce scale formation in water-carrying systems employed compounds such as tannins, modified lignins, algins, and other similar materials. Chelating or sequestering agents have also been employed to prevent precipitation or crystallization of scale-forming calcium carbonate. Another type of agent which has been actively explored heretofore as a calcium carbonate scale inhibiting material is the threshold active inhibitor. Such materials are effective as scale inhibitors in amounts considerably less than that stoichiometrically required, and this amount, as already mentioned, is termed the threshold amount. Inorganic polyphosphates have long been used as such threshold active inhibitors. For examples of such materials, see Fink--U.S. Pat. No. 2,358,222; Hatch--U.S. Pat. No. 2,539,305; and Ralston U.S. Pat. No. 3,434,969. Certain water soluble polymers, including groups derived from acrylamide and acrylic acid have been used to condition water containing scale-forming calcium carbonate. For example, see U.S. Pat. Nos. 2,783,200; 3,514,476; 2,980,610; 3,285,886; 3,463,730; 3,518,204; 3,928,196; 3,965,027; and 4,936,987In particular, there has been employed anionic polyelectrolytes such as polyacrylates, polymaleic anhydrides, copolymers of acrylates and sulfonates, and polymers of sulfonated styrenes. See, for example, U.S. Pat. Nos. 4,640,793; 4,650,591; 4,457,847; and 4,671,888. However, when used as threshold alkaline earth metal scale inhibitors, large dosages of these polymers are required, which in turn increases operating costs.
While various polycarboxylates, including polyacrylic acid, have been used as scale inhibiting agents, as described above, no similar use has been made of polycationic agents, apparently because of the difference in electronic charge and the conventional theories of the mechanisms of action for polymeric threshold inhibitors and dispersants.
Neither the N,N-bis(phosphonomethyl)-2-amino-1-propanol and corresponding lower alkyl ethers and N-oxides of the type which comprise the active ingredient of the compositions of the present invention are known, nor is their use for the control of alkaline earth metal scale, particularly calcium carbonate scale, under severe conditions which include elevated pH and high calcium carbonate saturation levels, with enhanced resistance to degradation by oxidizing biocides. Nevertheless, other phosphonates of related structure are known in the art for scale inhibition use.
For example, U.S. Pat. No. 4,080,375 discloses methylene phosphonates of amino-terminated oxyalkylates for use as scale inhibitors, but these compositions are not the same as those of the present invention, nor is there any suggestion that such compositions would be useful under severe conditions as defined herein, where phosphonates such as HEDP and AMP give poor results. U.S. Pat. No. 4,931,189 discloses aminomethylene phosphonates for inhibiting oil field scale formation involving a high brine environment susceptible to gypsum or barite scale formation. Such use in no way suggests the control of scale under the severe conditions described herein under which the compositions and methods of the present invention operate with surprising success.
A particular phosphonate which has been marketed for scale control, but apparently not suggested for use under the severe conditions defined herein, is ethanolamine N,N-dimethylene phosphonic acid, sold under such trademarks as WAYPLEX 61-A and BRIQUEST 221-50A.
U.S. Pat. Nos. 4,973,744 and 5,069,798 disclose N,N-bis-phosphonomethyl 2-(hydroxyethoxy) ethylamines and their N-oxides useful as scale inhibitors, but there is no suggestion of the unique isopropyloxy compounds of the present invention, nor of their particular usefulness for controlling scale under severe conditions.